1. Field of the Invention
The present invention relates to a semi-submersible offshore platform that has articulated buoyancy. More particularly, the present invention relates to a semi-submersible ocean platform having a submersible hull for oil storage that is attached to the platform by a fixed watertight column and having a set of articulated buoys.
2. Description of the Related Art
In an attempt to locate new oil fields, petroleum exploration expanded beyond land-based operations, with the petroleum industry seeking oil under the ocean and other bodies of water. At first, exploration, drilling, and production were generally conducted in the relatively shallow waters near land masses. In conventional shallow water drilling, a self-contained fixed platform is erected, the platform being supported by pilings driven into the sea floor. In deep water, however, such platforms cannot be used, because it is not feasible to erect a platform with pilings sufficiently long to stretch from the surface to the sea floor.
In deep water applications, therefore, the industry turned to semi-submersible, surface floating drilling vessels that are either towed or self-propelled to the drilling site. Semi-submersible drilling platforms include a submersible hull and an above-water deck that are interconnected. These semi-submersible platforms are moored over the drilling site and are normally provided with a central opening through which the drilling rig is operated.
Conventional semi-submersible drilling platforms, however, have various drawbacks. First, conventional semi-submersibles are generally rigid, with a minimum of moving parts, resulting in a lack of stability in response to substantial wave forces. In such rigid semi-submersibles, the hull and deck are rigidly attached, lacking additional stabilizers that can move with respect to the unitary hull/deck. Second, conventional semi-submersibles generally lack storage capacity in the hull; they must therefore be off-loaded to an interconnected tanker when pumping undersea oil to the surface, i.e., when in the production state.
To overcome the first drawback, i.e., lack of stability, semi-submersible platforms have been designed with buoys or stabilizers that move radially (i.e., pitch and roll) relative to the unitary hull/deck about a fixed point on the submerged hull. Examples of designs with buoys that pitch and roll appear in U.S. Pat. Nos. 3,837,309, 3,880,102, and 4,556,008. Such designs include several elongated buoys affixed to the perimeter of the hull, for example, by a universal joint. The addition of such buoys in these designs increases the stability of the platform in rough seas.
But these designs do not optimize stability, because the buoys lack the capacity to sway and surge or heave relative to the hull/deck and to each other. For purposes of this description, the terms "sway" and "surge" refer to side-to-side movement, in other words, movement along the plane of the water's surface. The term "heave" refers to up and down movement, that is, movement along a line connecting the sea floor and surface. A platform having buoys that can move radially about the platform and can also sway, surge and heave relative to the platform and to each other would substantially enhance the stability of the platform over one having buoys that merely move radially.
As to the second drawback, i.e., the lack of oil storage capacity in the hull, most semi-submersible drilling platforms lack any such storage capacity. Although some platforms have hulls with storage, no conventional platform exists incorporating buoys that can sway and surge or heave with hull storage. Furthermore, no platform exists having hull storage, a fixed centralized columnar support affixing the deck to the hull, and buoys that can sway and surge and/or heave. The fixed centralized support not only can support the deck, but also can provide access to the hull and serve as an enclosure for piping, machinery, etc. And with a fixed centralized support, the need for peripheral truss columns is eliminated, thereby reducing complexity and expense. Accordingly, no platform exists that features the advantages provided by buoys that can sway and surge and/or heave and a storage hull, much less such a storage hull having a fixed centralized columnar support member.
Therefore, a need exists for a semi-submersible deep-water platform that has a submersible hull for oil storage attached to the deck by a fixed watertight column and has a set of articulated buoys that can heave, sway and surge relative to the platform and relative to each other.